The monitoring of concentrations of specific gases and vapours, particularly very low concentrations of such gases and vapours, is becoming increasingly important in many applications, particularly in pollution control and other environmental monitoring operations. A number of techniques have been employed or proposed for such applications. An ideal instrument for field use has sufficient sensitivity to respond to very low gas or vapour concentrations, a fast response time, and sufficient robustness to be utilized in the field. The object of the present invention is to provide an instrument which can meet these requirements.
Absorption spectrometers are well known and have previously been utilized for monitoring gas concentrations. One type of such spectrometer particularly suited to the monitoring of low concentrations of specific gases is the correlation type spectrometer in which the transmission of light, usually ultraviolet light, through a gas sample being monitored at a predetermined set of wavelengths corresponding to the absorption maxima of the gas being monitored is compared with that of light at a set of wavelengths displaced from the absorption maxima, any difference detected providing a sensitive indication of the presence of the gas being monitored and its concentration, since at low concentration the absorption by a gas is proportional to its concentration. In such intruments, light passed along an absorption path through gas being monitored is analyzed in a spectrometer and the desired wavelengths are isolated by means of a slotted mask or masks.
In one arrangement described in U.S. Pat. No. 3,518,002 issued June 30, 1970 to Barringer, the spectrum of the dispersed light passed through the sample is caused to fall on a slotted mask and is oscillated relative thereto by a vibrating refractor, so that the slots in the mask move cyclically into and out of correlation with the wavelengths corresponding to the absorption maxima of the gas being analyzed. In other proposed arrangements, the mask itself is vibrated.
In U.S. Pat. No. 3,837,744, issued to Egan et al, the light passed through the absorption cell is divided in two and caused by a radiation chopper to pass alternately along different paths through different slotted masks. In U.S. Pat. No. 4,057,734 issued Nov. 8, 1977 to Barringer, a radiation chopper is again used to pass the light from the absorption cell alternately along two different paths. Both these systems require duplication of many of the optical components of the system.
A correlation spectrometer is marketed by Lear Siegler Incorporated which differs from the foregoing in that the light is restricted to a set of required wavelengths prior to its passage through the absorption cell, and the rate of change of absorption by the cell as the wavelengths are swept is sensed so as to detect the gas being monitored. The prior restriction of the wavelengths present in the cell reduces spurious outputs.
All of the above systems rely on mechanical means to achieve the required correlation action by chopping or oscillating between the different sets of wavelengths at which absorption is being compared. I have found that at low gas concentrations, the sensitivity of the systems is limited by mechanical "noise" or jitter, and the reduction of such jitter presents particular problems in systems for field use, which may have to be used in environments on vehicles or aircraft which subject the equipment to vibration and other stresses. Existing systems have inevitably involved a compromise between robustness and mechanical precision.